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Abstract: This paper studies Al–steel lap joints resulted from direct-current pulsed gas metal arc welding (DPG) and alternate-current double-pulse gas metal arc welding (ADG). Base metals being welded are 1mm thick 5052 Al alloy sheet and 1mm thick galvanized mild steel sheet with filler metal being 1.2mm 4047. Effect of the different welding processes on joint properties is studied through characterization of the steel/Al interphases and mechanical strength. It is found that the thickness of intermetallic compound layer composed of Fe2Al5, FeAl3and Fe3Al, and the diffusion of Fe element from steel to weld seam are dependent on the amount of welding heat input. Lap shear samples of Al–Fe joints resulted from the ADG process fractured at Al base metal or weld seam with average ultimate tensile strength (UTS) of 201MPa. On the other hand, all lap shear samples of the joints resulted from DPG process fractured along the interphase between the Fe2Al5 sub-layer and the steel sheet with average UTS of 115MPa. ADG process offers higher joint strength than that offered by DPG process due to thinner intermetallic compounds layer caused by the reduced heat input, illustrating that cyclic polarity switching and a secondary low frequency current pulse can effectively reduce the heat input in joining Fe–Al dissimilar materials, which means the ADG process is much more feasible for making robust Fe–Al lap joints. [Copyright &y& Elsevier]

In this paper, the microstructure optimization and mechanical property of joints are studied when the Cu, CuSi3, and CuNi10 wires are used to Cold Metal Transfer arc-braze TC4 to 304L. Based on the microstructure analysis and thermodynamic calculation, the existing state of Ti and Fe atoms are transferred from high brittleness Ti-Fe to less brittle Ti-Fe-Si intermetallics when the welding wire is changed from Cu to CuSi3 due to the lower Gibbs free energy of Ti-Fe-Si intermetallics. Furthermore, the (Fe, Ni) solution and Ti-Ni intermetallics with the lowest hardness are formed when CuNi10 wire is used. Thus, three Ti-Fe suppression mechanisms are concluded: suppressed by forming intermetallics and diffusion inhibition, suppressed by forming intermetallics, and suppressed by forming intermetallics and solution. Based on the microstructure optimization, the max hardness of intermetallics layer decreases from 769 to 654 Hv and the max tensile strength increases from 186.4 to 319.4 MPa. However, all cracks of the joints with different wires will initiate from the bottom of the seam and propagate in the TC4/seam transition zone due to the high Schmid factor. Besides, the crack propagation rate of more than 200 m/s can be in-situ measured by high-speed camera with 200000 fps. • Si addition can transfer Ti-Fe into Ti-Fe-Si intermetallics. • Ni addition can transfer Ti-Fe into Ti-Ni intermetallics. • The crack initiation and propagation are in-situ observed by High speed camera. [ABSTRACT FROM AUTHOR]

Due to low density and good high temperature performance, titanium aluminide has been continuously attractive for aerospace and automotive industries. In recent years, additive manufacturing of titanium aluminide has become a popular research area. The present work has first time fabricated commercial graded TiAl-4822 alloy using an innovative twin-wire directed energy deposition-arc (TW-DED-arc) process, which uses a unique twin-wire feeding method to produce the target alloy in-situ. According to experimental results, TW-DED-arc fabricated TiAl-4822 alloy possesses direct full density, homogeneous chemical composition, uniformed microstructure, and stable mechanical property. Also, the formation mechanism of layered microstructure consisting of alternative columnar and equiaxed grain regions has been investigated in detail. Even under as-fabricated condition, TW-DED-arc fabricated TiAl-4822 performs competitive tensile properties compared to the same alloy produced by casting and other additive manufacturing methods. Generally, successful TW-DED-arc buildup of TiAl-4822 provides a high-efficiency and low-cost route for the further development of titanium aluminide production. [ABSTRACT FROM AUTHOR]

Due to the intrinsic high room temperature brittleness and cold-cracking susceptibility, the fabrication and forming of γ-TiAl intermetallic alloy component is extremely difficult. Therefore, in recent years, a wire-arc additive manufacturing (WAAM) technique has been developed to fabricate the γ-TiAl intermetallic alloy by depositing the Ti and Al wires into a single tungsten arc generated molten pool with specific wire feed ratios. However, the WAAM fabricated γ-TiAl intermetallic alloy has been found having inhomogeneous layer-by-layer microstructure and the excessive heat input of tungsten arc would induce significant residual stress in the bulk sample. In the present paper, the previous WAAM has been further upgraded and an innovative twin-wire plasma arc additive manufacturing (TW-PAAM) process has been developed. Afterwards, a γ-TiAl intermetallic alloy wall component with specific chemical composition of Ti-48Al has been fabricated and the metallography, phase composition and tensile properties are characterized subsequently. It has been found that a significantly more uniform microstructure is obtained in the TW-PAAM fabricated γ-TiAl intermetallic alloy than the previous WAAM technique. The content of α 2 phase, lamellar colony size and lamellar spacing exhibited the tendency of decreasing from the lower to upper part along building direction. And the tensile strength and ductility of the lower section are lower than the middle and top sections. In general, the present TW-PAAM technique has shown promising capability of fabricating γ-TiAl intermetallic alloy with lower cost, and the investigation results would become a valuable reference for understanding the evolution mechanism of microstructure and mechanical properties of the additively manufactured TiAl alloy. [ABSTRACT FROM AUTHOR]

Considering the advantages of low density and excellent high-temperature performance, titanium aluminide is identified as an ideal aerospace structural material. In recent years, twin-wire directed energy deposition-arc (TW-DED-arc), which possesses the characteristics of low cost but high deposition efficiency, has continuously attracted worldwide attention. However, obtaining equiaxed grains with small Al variation for titanium aluminide using TW-DED-arc method is still a key concern. In the present research, Ti–48Al–2Cr–2Nb alloy with equiaxed lamellar colonies and less than 1 at.% of Al content was successfully fabricated by regulating deposition current of TW-DED-arc. Also, the effect of deposition current on microstructure and mechanical properties was systematically investigated. The experimental results that focus on top and middle regions of as-fabricated TiAl-4822 alloys, indicate that increasing current favors in generation of equiaxed grains and γ lamellae, which leads to columnar to equiaxed transition and higher γ phase content, respectively. Also, elemental homogeneity can be significantly homogenized. As deposition current increases, tensile strength increases while degree of tensile anisotropy decreases significantly, benefiting from the equiaxed grains with satisfying size. Importantly, microstructure evolution, tensile anisotropy and fracture characteristics of TW-DED-arc fabricated TiAl-4822 alloys are discussed in detail. Generally, these findings provide an effective process method to optimize microstructure and mechanical properties of TW-DED-arc fabricated TiAl-4822 alloys. [ABSTRACT FROM AUTHOR]

Titanium aluminide has continuously attracted the attention of aerospace industry due to its low density and good performance at high temperatures. To overcome the room temperature brittleness induced manufacturing difficulty, additive manufacturing (AM) technology of titanium aluminide has been developing rapidly since last decade. Compared to powder-based AM methods such as electron beam melting (EBM), the twin-wire directed energy deposition-arc (TW-DED-arc) method for titanium aluminide fabrication possesses outstanding material deposition efficiency and much lower cost. However, because of the excessive arc deposition energy, the layer-band like microstructure of TW-DED-arc fabricated titanium aluminide is a great concern affecting the mechanical properties. In the present research, microstructure of as-fabricated TiAl alloy is modified by adjusting the deposition current of TW-DED-arc. Experimental results show that along with the increase of deposition current, the layer-band like microstructure consisting of dendritic grain region and fully lamellar colony region can be mostly homogenized. Although microhardness is decreased because of the inevitable lamella growth, tensile strength of TiAl deposit is on the other hand increased by eliminating the interdendritic γ-phase. In general, the present work has proved that deposition current adjustment is an effective method for homogenizing the microstructure of additively manufactured titanium aluminide. [ABSTRACT FROM AUTHOR]

TiAl alloys are considered as promising high temperature structural materials. However, the inherent brittleness makes it difficult to be processed. In the present research, TiAl alloys with different Al content are fabricated successfully using an innovative twin-wire plasma arc additive manufacturing system. The effect of Al level on the phase composition, microstructure characteristics, microhardness and tensile properties of as-deposited TiAl alloy is investigated in detail. The results show that the α 2 phase content exhibits increase tendency with the decrease of Al level, and the α 2 phase content is higher in the top region than that in the middle region. The tetragonal ratio c/a of γ phase tends to reduce with the decrease of Al content. The microstructure characteristics also present obvious difference for as-deposited TiAl alloys. The lamellar spacing gradually decreases with the decrease of Al concentration. With decreasing Al level, the microhardness of alloy tends to increase. In addition, the as-deposited Ti-48 Al (at.%) alloy exhibits much better tensile properties than other TiAl alloys due to optimum microstructure characteristics. The findings provide a valuable reference for additively manufactured TiAl alloy with proper composition ratio. [ABSTRACT FROM AUTHOR]